In many technical fields EMI (electromagnetic interference) is a constraint. For example in automotive vehicles, the electromagnetic environment generated by devices like ABS braking systems, airbag sensors, or engine management, is omnipresent. Hence, EMC (electromagnetic compatibility) is of great importance to safety. Without a proper design, EMC of integrated circuits will become a limiting factor in the performance of every advanced electronic system. However, due to more sizable wires forming antennas and to more rapidly changing currents, the current variation (di/dt) in power supply lines generates most of the noise radiation emitted. Therefore, making di/dt values smaller is necessary in order to reduce the EMC problem at the chip-level.
It is known to use a capacitor coupled between the supply lines on board the chip, or if too large to be integrated, then located next to the chip. It is also known from U.S. Pat. No. 6,489,815 that in buffer circuits having first and second current sources connected to different supply lines, to produce a switched output, depending on use of a termination resistor, a large constant current varies upon switching, which increases radiation noise. To suppress such noise by suppressing a variation of the current, a low-noise buffer circuit according to this document has a resistor connected between the first and second current sources.
U.S. Patent Application Publication No. 2004/0257053 has a power supply circuit which comprises a differential amplifier for feeding out a voltage as a control voltage in accordance with a difference between a feedback voltage commensurate with an output voltage and a reference voltage, and an output current control element for feeding out an output current in accordance with the control voltage fed thereto from the differential amplifier. A voltage on the output line is fed back as the feedback voltage to the differential amplifier, the feedback line is connected to an output line, and a constant current source, connected between the feedback line and ground, is provided for generating a sink current by which the control voltage fed to the output current control element is raised to a predetermined value.
For a configuration in which a capacitor is connected between the output line and ground, at least the electrical charge stored in the capacitor is removed when the switching element is turned on. Therefore, when the load changes from a heavy load to no load or to a light load and when the output voltage is raised due to a transient phenomenon, the electrical charge stored in the capacitor is removed swiftly. This makes it possible to further suppress the fluctuations of the output voltage caused by load fluctuations, and improve properties in the transient response.
When the load changes from no load to a heavy load, the gate voltage of the output current control element, is raised by the sink current Ic that is pulled in by the constant current source during the no-load period preceding the change in load condition. This makes it possible for the current control element to respond faster, thereby reduce fluctuations of the output voltage caused by fluctuating load conditions to a minimum, and, in addition, improve properties in the transient response. However, this will tend to worsen the rate of change of current drawn by the current control element.